Preheat oven to 325F/160C (standard oven / fan forced or convection)
Butter a 8” x 8” / 20cm x 20cm square cake tin (Note 4).
Beat egg whites with a mixer until stiff peaks form. Set aside.
Place the egg yolks and sugar in a bowl and beat until it turns pale yellow - about 1 minute.
Add the vanilla extract and butter and beat until well incorporated - about 30 seconds to 1 minute.
Add the flour and beat until just combined.
Pour the milk in slowly while beating, and beat until well combined (or if using a hand held mixer, add 1/4 milk at a time, beating in between).
Use a spatula to fold in the egg whites in the batter, one third at a time, until just incorporated. You don’t want to knock the air out of the egg whites. Don’t worry if there are a few egg white lumps in the batter. The batter should be very thin, almost like a thick pouring cream.
Pour the batter into the prepared cake tin.
Bake until the top is golden brown and the cake does not “jiggle” when you gently shake the tin - around 40 to 50 minutes. Check the cake at 30 minutes - if the top is already golden brown but the cake is not yet set (i.e. it jiggles), cover loosely with foil and return to the oven, 10 minutes at a time, until set.
Allow to cool in the tin for 10 minutes, then turn out onto a cooling rack.(Note 3) Cool completely before serving.
Cut into 9 squares to serve.
Dust with icing sugar just before serving.
Serve with a side of whipped cream and strawberries, if using.